1. Field of the Invention
The present invention is directed to polyisocyanates which are based on hexamethylene diisocyanate, are liquid and have improved dispersibility.
2. Background of the Invention
Water dispersible polyisocyanates prepared from polyisocyanates having aromatically bound isocyanate groups are known and disclosed in U.S. Pat. No. 4,433,095. Water dispersible polyisocyanates prepared from polyisocyanates containing (cyclo)aliphatically bound isocyanate groups are known and have been disclosed in U.S. Pat. No. 4,663,377; German Offenlegungsschrift 3,829,587 (U.S. Ser. No. 07/395,849); and U.S. Ser. Nos. 07/644,180, 07/643,615 and 07/644,798. These polyisocyanates are generally rendered hydrophilic by reaction with a monohydroxy functional polyether based on ethylene oxide or a mixture of ethylene oxide and propylene oxide.
U.S. Pat. No. 4,472,550 is directed to the use of monohydroxy functional polyethers which are initiated with specific cyclic compounds for providing hydrophilicity to both polyisocyanates having aromatically bound isocyanate groups and polyisocyanates having (cyclo)aliphatically bound isocyanate groups.
Even though the monohydroxy functional polyethers are effective for rendering polyisocyanates having (cyclo)-aliphatically bound isocyanate groups water dispersible, it is often necessary to dissolve the polyisocyanate in an organic solvent in order to assist its dispersion in water. In addition, the stability of the dispersed polyisocyanates needs to be improved. Accordingly, there is a need to further improve the hydrophilicity of the polyisocyanates and either reduce or eliminate the need for the polyisocyanate to be dispersed in an organic solvent during the dispersion step.
Another difficulty of the prior art monohydroxy functional polyethers based exclusively on ethylene oxide is that these polyethers are solid and often result in a modified polyisocyanate which also is solid. Accordingly, prior to dispersing these modified polyisocyanates in water it is necessary to dissolve them in an organic solvent and add them to water in solution. While it is possible when using blocked polyisocyanates to remove the solvent after the polyisocyanates are dispersed in water, it is not possible to remove the solvent from unblocked polyisocyanate dispersions. Because the dispersed polyisocyanates have a limited potlife, they cannot withstand the time or the elevated temperatures which are necessary to remove the solvent.
In an effort to liquify the solid polyethers and avoid the necessity of using organic solvents, the prior art references previously mentioned often use a monohydroxy functional polyether prepared from both ethylene oxide and propylene oxide. While such polyethers are liquid the presence of propylene oxide adds weight to the polyether and, thus, reduces the isocyanate content of the modified polyisocyanates. In addition, because propylene oxide is not hydrophilic like ethylene oxide, additional ethylene oxide is necessary to provide hydrophilicity to the propylene oxide, and this also reduces the isocyanate content of the modified polyisocyanates.
Accordingly, it is an object of the present invention to provide modified polyisocyanates based on hexamethylene diisocyanate which are liquid, do not require the use of organic solvents for dispersion in water and have improved hydrophilicity when compared to prior art liquid, modified polyisocyanates.
This object may be achieved in accordance with the present invention as described hereinafter by reacting a polyisocyanate adduct prepared from hexamethylene diisocyanate either alone or in admixture with other diisocyanates with a low molecular weight monohydroxy functional polyether prepared from ethylene oxide.
Even though U.S. Pat. No. 3,996,154 and British Patents 1,417,618 and 1,444,933 disclose the use of low molecular monohydroxy functional polyethers prepared from ethylene oxide for rendering polyisocyanates water dispersible. These references do not disclose polyisocyanate adducts prepared from hexamethylene diisocyanate as suitable polyisocyanates for modification with the polyethers. These references are primarily directed to the modification of polyisocyanates containing aromatically bound isocyanate groups such as toluene diisocyanate, diphenylmethane diisocyanate and polyphenyl polymethylene polyisocyanates. In addition, the examples set forth hereinafter demonstrate that the improvements in hydrophilicity obtained for the claimed polyisocyanate compositions are not obtained when using polyphenyl polymethylene polyisocyanates.